Solid lubricant and method of making the same

ABSTRACT

A solid-state lubricant composition is disclosed. The solid-lubricant contains graphene, an oxide of a metal, and one or more polymeric binders. A method of making a solid-state lubricant coating on various substrates is disclosed. The method includes the steps of making a homogeneous slurry comprising powder of an oxide of a metal, graphene, and a polymeric binder with organic volatile solvent; coating a substrate with the homogeneous slurry with desired thicknesses; and drying the slurry on the substrate naturally or applying additional heat, resulting in a solid lubricant coating on the substrate. Substrates with coated solid composite lubricant show wear reduction and lower coefficient of friction compared with uncoated substrates.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present U.S. patent application is a divisional of U.S. patentapplication Ser. No. 16/013,441 filed Jun. 20, 2018, which is related toand claims the priority benefit of U.S. Provisional Patent ApplicationSer. No. 62/523,707 filed Jun. 22, 2017, the contents of which arehereby incorporated by reference in their entirety into the presentdisclosure.

TECHNICAL FIELD

This disclosure relates to dry solid lubricants, especially compositelubricants containing graphene, and methods of making them.

BACKGROUND

This section introduces aspects that may help facilitate a betterunderstanding of the disclosure. Accordingly, these statements are to beread in this light and are not to be understood as admissions about whatis or is not prior art.

Mechanical systems with greater energy efficiency and lower environmentimpact require enhanced performance at moving interfaces. Thefundamental causes of mechanical failure are friction and wear: reducingthese energy losses improves performance and lifetime of many mechanicalsystems. Conventional methods to improve energy efficiency utilizeliquid lubricants, including organic oils, to reduce friction betweencontacting surfaces in relative motion. Furthermore, the addition ofnanoparticles to lubricating oils has been shown to further enhancetribological performance. However, despite their convenience andutility, liquid lubricants cannot be used in situations of hightemperature or low pressure (viz., vacuum), due to the risk ofvolatilization and fire. In this context, dry solid-state lubricants area viable alternative to their liquid counterparts in extreme operatingenvironments.

Graphene, due to its distinct material properties, is a promisingcandidate for solid-state lubrication. Graphene is a unique carbonallotrope, comprising a flat lattice of aromatic carbon rings only oneatom thick. This two-dimensional arrangement enables superior thermalconductivity, extreme mechanical strength, and ultralow friction. Inparticular, the low coefficient of friction for graphene materials hasbeen demonstrated at the nanoscale by atomic force microscopy. Theseuncommon properties, combined with continuous production processing,distinguish graphene from other nanomaterials for friction and wearreduction. However, existing tribological studies of graphene at themicroscale and macroscale show that friction and wear reductions onlyoccur under low contact pressures: friction and wear rapidly increaseunder high contact pressure (i.e., >0.5 GPa). The failure of baregraphene is attributed to poor adhesion with substrate surface, enablinggraphene ejection under excessive pressure. Therefore, durable adhesionbetween the lubricant film and contact surfaces is critical forapplication of graphene as a solid lubricant.

Thus, there exists an unmet need for solid lubricants containinggraphene which are capable of excellent adhesion to contact surfaces.

SUMMARY

A solid-state lubricant composition is disclosed. The solid-lubricantcontains graphene, an oxide of a metal, and a polymeric binder.

A method of making a solid-state lubricant coating on a substrate isdisclosed. The method includes the steps of making a homogeneous slurrycomprising powder of an oxide of a metal, graphene, a polymeric binderan organic volatile solvent; coating a substrate with the homogeneousslurry; and drying the slurry on the substrate, resulting in a solidlubricant coating on the substrate.

BRIEF DESCRIPTION OF DRAWINGS

Some of the figures shown herein may include dimensions. Further, someof the figures shown herein may have been created from scaled drawingsor from photographs that are scalable. It is understood that suchdimensions, or the relative scaling within a figure are by way ofexample, and not to be construed as limiting.

FIGS. 1A and 1B depict thermogravimetric analysis of zinc acetateshowing initial mass losses and greatest mass loss respectively.

FIG. 2A shows X-ray powder diffraction of the composite coating of thisdisclosure.

FIG. 2B shows Raman spectroscopy of the composite coating of thisdisclosure.

FIG. 3A is a scanning electron micrograph of the composite of thisdisclosure.

FIGS. 3B and 3C show energy-dispersive spectroscopy of carbon contentand zinc content respectively of the composite coating of thisdisclosure.

FIG. 4 shows friction reduction resulting from the composite coating ofthis disclosure.

FIG. 5 shows the effect of film composition on friction reduction.

FIG. 6 shows optical micrographs of wear tracks are dependent on thecomposition of the lubricant used.

FIG. 7 shows effect of applied load on friction reduction. The steadystate coefficient of friction for the composite coating is constantunder an applied load up to 15 N.

FIG. 8 shows effect of sliding distance on friction reduction for thecomposite coating of this disclosure.

FIG. 9 shows 3D surface reconstruction of sliding distance-dependentwear tracks of the moving (rotating) specimens in the tribology testing.

FIG. 10 shows 3D surface reconstructions of composition-dependent weartracks on the rotating specimens in tribology testing.

FIGS. 11A through 11D show Raman spectral maps of stationary surfacespecimen.

FIGS. 12A and 12B show Raman spectral map of disk substrate surface attwo different locations

FIG. 13 is a schematic representation of synthesis of zinc oxide,graphene, and binder composite film of this disclosure.

FIG. 14 shows a comparison of the coefficient of friction of a surfaceobtained by using a composite coating of this disclosure utilizingAremco-Bond™ 570 product (containing butanone, isopropanol, phenol,carbon black, formaldehyde, and o-cresol) as a binder in the compositecoating, and an unlubricated surface.

FIG. 15 shows effect of normal load conditions on the coefficient offriction of a surface coated with a composite coating of thisdisclosure.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thedisclosure, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of thedisclosure is thereby intended.

In this disclosure, a graphene-rich composite is studied as a solidlubricant to minimize friction and wear losses under high contactpressures and long sliding distances. One example of the composite ofthis disclosure comprises graphene, zinc oxide, and polyvinylidenedifluoride binder. In this disclosure, such composites are termed“composite coating” or “composite lubricant” interchangeably. In thisdisclosure the word “composite” or the phrases “composite coating” or“composite lubricant” or “composite film” or “composite lubricant film”are used to signify that there is at least one other ingredient otherthan graphene in the lubricant. Further, in this disclosure, the terms“solid lubricant”, “solid state lubricant” and “solid-state lubricant”are used interchangeably. In some instances, the phrase “dry solidlubricant” is used to indicate the solid lubricant. Thus dry solidlubricant and solid lubricant and dry lubricant are also usedinterchangeably in this description. Thus the solid lubricants of thisdisclosure are composite lubricants or composite coatings. Inexperiments leading to this disclosure spin coating was employed toapply the graphene-based composite as an approximately 10 μm thick filmonto a stainless steel surface. Tribological performance was measured inthe ball-on-disk configuration under ambient conditions. It was foundthat the composite film significantly improves friction and wearreduction (approximately 90%) relative to unlubricated contact.Following tribological testing, Raman spectroscopic analysis of producedwear tracks revealed a persistent protective film on both contactsurfaces. It is theorized that zinc oxide enables durable binding ofgraphene to the contact surfaces, enabling friction and wear reductionunder the unusual conditions of high contact pressure inside the contactarea. It should be noted that those skilled in the art will recognizethe nomenclature used in this description both for the test parameters(such as sliding distance and stationary specimen or sample) and thevocabulary used in reporting of the results.

Primarily, the lubricant is made in a liquid slurry form comprising allingredients of the composite, which converts into dry form afterlaminating on mechanical parts followed by evaporating organic solvent.The applied dry lubricant can be cured (heat treatment at less than 150Degree Celsius) to enhance the adhesion properties. Finally, it remainsas a solid material on the applied mechanical flat (two dimensional) orone dimensional or 3 dimensional surfaces termed as solid or dry(without solvent) lubricant. The initial liquid slurry can be sprayed asan aerosol, spin coated or laminated applying doctor blade method on themechanical surfaces to develop a solid lubricant.

Formation of Graphene-Based Composite: The composite film of thisdisclosure comprises graphene, zinc oxide, and polyvinylidene fluoride(PVDF). It is believed that the primary lubrication effects are due tographene, while synergistic adhesion is due to zinc oxide and PVDF.Specifically, zinc oxide provides intraphase adhesion (that is, betweenthe composite coating and the substrate surface) while PVDF providesinterphase adhesion (within the composite coating). Characterization ofthis system has been performed to understand the chemical and materialproperties of the film relative to its constituents.

In this detailed description the material properties, characterizationof the materials and tribological testing results of the solid lubricantof this disclosure are presented first. In the later sections, themethod of making the solid lubricant, the characterization methodsemployed for the ingredient materials and the composite and thetribological testing procedures are detailed.

In experiments leading to this disclosure, Zinc oxide was synthesizedfrom zinc acetate dihydrate by thermal decomposition. Thermogravimetricanalysis indicated that the macroscale mechanism is comprised of twoindependent steps as shown in FIGS. 1A and 1B. FIG. 1A depicts the firststep of thermogravimetric analysis of zinc acetate showing that thefirst mass loss of 15.6%-wt. occurs at temperature 117° C. due tovaporization of hygroscopic adsorbed moisture. FIG. 1B depictsthermogravimetric analysis of zinc acetate showing that the second massloss of 71.6%-wt. occurs at temperature 316° C. due to the thermaldecomposition of the acetate anion. The broad, overlapping features ofthe differential thermogram near 316° C., shown in FIG. 1A suggests thisdecomposition includes several elementary steps. The overall mass lossof this reaction is 87.2%-wt., producing a product yield of 12.8%-wt.

The prepared composite coating is considered a physical mixture of itscomponents. FIG. 2A shows X-ray powder diffraction of the compositecoating, showing that preparation of the composite film does notchemically change the identity of the film precursors. Referring to FIG.2A, the characteristic x-ray spectral features of the dried compositecoating are identical to those of its constituent components. Further,X-ray powder diffraction (XRD), shown in FIG. 2A, demonstrated that thecrystallographic profile for the composite is a linear combination ofthe profiles for the components. The synthesized zinc oxide exhibitssharp, spectral features of hexagonal zincite. Graphene exhibitsspectral features representing the 002 and 100 facets of hexagonalcarbon. The additional features near scattering angles 22, 42 and 44°respectively represent the 009, 101, and 015 facets. These secondaryfeatures are characteristic of nitrate functionalization, a commonconsequence of graphene production through chemical exfoliation. PVDFexhibits diffuse spectral features, indicative of disordered materials,that represent a mixture of the α and β phase. In comparison, thescattering spectrum of the composite is comprised of the principleintensity features of its constituents. The broad, shallow, andirregular baseline that occurs between 12°-42° is due to the lowintensity of PVDF relative to the more crystalline components. Theintense features that occur near 23, 44, and 50° correspond to the highcrystallinity of graphene. The remaining spectral features correspond tothe crystal facets of zinc oxide. The spectral features of the compositeX-ray spectrum are a direct consequence of the discrete nature of thecomposite film.

FIG. 2B shows Raman spectroscopy of the composite coating of thisdisclosure. Referring to FIG. 2B, the Raman spectral features of thedried composite coating only match those of graphene, which demonstratesan overwhelming Raman excitation response. Raman spectroscopy also showsthat the binding of carbon within graphene remains unchanged from thefilm preparation process, as shown in FIG. 2B. Graphene exhibits intenseRaman excitation at frequencies 1332, 1577, and 2667 cm⁻¹ correspondingto vibrational modes D, G, and 2D, respectively. The areal intensityratio of the D and G peaks is calculated as 0.942 for graphene and 1.073for the composite film: high similarity in these values suggest thedistribution of sp² and sp^(a) hybridization of graphene is conserved inthe composite film. It is important to note that the spectral featuresof PVDF and ZnO are absent in the composite spectrum because thesefeatures produce negligible Raman intensity compared to those ofgraphene. Therefore, the presence of the composite film can be relatedto intense excitation of the D or G vibrational modes (FIG. 2B).

FIG. 3A is a scanning electron micrograph of the composite of thisdisclosure. Referring to FIG. 3A, the composite film contains a uniformdistribution of zinc oxide and graphene showing a homogeneousdistribution of microscale sheet-like particles and nano-scaleaggregates. Scanning electron micrographs have shown that the film isprimarily comprised of broad micro-scale graphene particles. FIGS. 3Band 3C show energy-dispersive spectroscopy of carbon content and zinccontent respectively of the composite coating of this disclosure.Referring to FIG. 3B element pixel maps from energy dispersive X-rayspectroscopy (EDS) confirm the high carbon content of the planargraphene particles, suggesting that the micro-scale particles areattributed to graphene. Furthermore, in FIG. 3C smaller nano-scaleparticles are observed across the basal graphene surfaces; these highzinc content particles, shown in FIG. 3C are zinc oxide. Thus, scanningelectron micrography and energy-dispersive x-ray spectroscopy of thecomposite coating demonstrated a homogeneous distribution of zinc withina carbon-rich graphene phase.

From the above characterization, the composite film of this disclosurecan be interpreted as a mono-disperse mixture of zinc oxide and PVDFamong a random arrangement of graphene. Zinc oxide and PVDF arenano-scale particles, while graphene is a micro-scale particle. Finally,it is observed that the applied preparation techniques do not chemicallymodify graphene, PVDF, or zinc oxide: rather, each component retains itscrystallographic order in the composite following preparation.

At the steel-steel point interface, the tribological performance of thegraphene-rich lubricant is superior to the performance in unlubricatedcontact. Under normal load of 10 N (Hertz contact pressure 0.89 GPa) tosliding distance of 145 m, the graphene-based composite demonstratedapproximately 90% friction reduction relative to unlubricated contact.This can be seen in FIG. 4 which shows friction reduction resulting fromthe composite coating of this disclosure. Referring to FIG. 4,unlubricated sliding results in a large coefficient of friction thatpersists over a long sliding distance. In comparison, lubricated slidingresults in a drastically reduced coefficient of friction across the samesliding distance. In FIG. 4 and several other figures where coefficientof friction is reported, the bands (data points) in the graphs representthe scatter or deviation in the data based on the coating thicknessdeviation, which is the result of collecting numerous data points. Thoseskilled in the art will readily recognize this.

During unlubricated steel-steel sliding, the coefficient of frictionerratically increases from approximately 0.20 to 0.90 after 30 m withunsteady behavior. This irregular behavior is attributed to thegeneration of wear particles at the sliding surfaces. Duringgraphene-lubricated sliding to 145 m, in contrast, the coefficient offriction reaches a maximum of approximately 0.13 and steadily decreasesto stability at 0.08 with stable and steady behavior during the entiretribo-test.

The role of zinc oxide in the graphene-based composite was investigatedby testing alternative coating compositions: (I) graphene, zinc oxide,and PVDF binder; (II) graphene and PVDF binder; and (III) graphene only.The graphene only composition measures the native friction reduction ofgraphene alone, while the graphene/PVDF composition measures frictionreduction of graphene in the presence of PVDF. FIG. 5 shows the effectof film composition on friction reduction. While graphene showsexceptional lubrication properties alone, zinc oxide critically reducesthe measured friction in the composite coating. Zinc oxide alsomitigates the increased friction resulting from the addition of PVDF tographene. Referring to FIG. 5, under applied normal load of 10 N to 145m sliding distance, the graphene/PVDF/ZnO composite demonstrates thegreatest friction reduction. The equilibrium coefficients of frictionwere measured as 0.08 for the total composite, 0.14 for graphene andPVDF, and 0.10 for graphene only. The friction coefficient for thegraphene/PVDF composite is greater than that of graphene alone, whichcould be due to the additional friction from the rough polymer. Incomparison, the friction coefficient for the graphene/PVDF/ZnO compositeis less than that of graphene alone, suggesting that zinc oxidesimultaneously enhances the native friction reduction of graphene andmitigates friction introduced by PVDF.

FIG. 6 shows optical micrographs of composition-dependent wear tracks onthe moving rotating disc specimen in the tribology testing. Thenomenclature used in this specification with reference to tribologytesting (e.g. stationary specimen) is well known to those skilled in theart. These micrographs show that dry lubricants improve the wearresistance, and that the presence of zinc oxide prevents formation of anappreciable track. Micrographs a and e show that unlubricated slidingproduces a deep, rough wear track. Micrographs b and f show thatlubricated sliding with graphene produces a smaller, smoother weartrack. Micrographs c and g show that lubricated sliding with grapheneand PVDF produces an even smaller, smoother wear track. Micrographs dand h show that the composite coating containing graphene, PDVF, andzinc oxide prevents formation of an appreciable wear track, generallypreserving the specimen surface. Thus, the composite coatings of thisdisclosure containing zinc oxide help binding graphene to the mechanicalsurfaces such as stainless steel via covalent or van der Waals bondsmaking stronger adhesion (tribofilm) yielding low friction and wear.Without ZnO the adhesion is weak and tribological properties are poor.

In addition to reducing friction, the composite coating reduces bothwear track formation and surface roughening. After testing with normalload 10 N to sliding distance of 145 m, optical micrographs and 3Dsurface scans of the ball specimen illustrate that the wear trackdecreases with the addition of components to the composite coating asshown in FIG. 6. After unlubricated sliding, the formed wear track hasdiameter 656 μm and roughness R_(a) 540 nm. After lubricated slidingwith graphene only, the wear track diameter and roughness are reduced to221 μm and 87 nm, respectively. After lubricated sliding with grapheneand PVDF, the wear track diameter and roughness are further reduced to158 μm and 72 nm, respectively. In this case, enhanced wear reduction isattributed to the increased coating strength from PVDF. Finally, afterlubricated sliding with graphene, zinc oxide, and PVDF, an appreciablewear track was not formed: the specimen surface demonstrates only a fewdiscernable scratches. In fact, a dark opaque film is observed in placeof a wear track as shown in micrograph designated as “d” in FIG. 6. Itis expected that this residual coating enhances friction and wearreduction by preventing direct contact to the sliding disk surface.

The composition of the coating of this disclosure significantly improvestribological performance. Compared to unlubricated steel-steel sliding,lubricated sliding with graphene alone results in drastic frictionreduction to 10 N load and 145 m sliding distance tribo-test. Theaddition of PVDF to the film also reduces wear and friction compared tounlubricated sliding, however, friction reduction is greater than thatof graphene alone. The addition of zinc oxide to the composite filmreduces friction below that measured in either lubricant compositions.These findings suggest the importance of zinc oxide to the compositionof this adhesive composite.

To quantify durability of the composite coating, the coefficient offriction was measured under normal loads 5, 10, and 15 N, correspondingto Hertz contact pressures of 0.71, 0.89, and 1.02 GPa, respectively.FIG. 7 shows effect of applied load on friction reduction. Referring toFIG. 7, the steady state coefficient of friction for the compositecoating is constant under an applied load up to 15 N. After extendedtime in ball-on-disk sliding, the coefficient of friction of thecomposite coating remains constant. In all trials, the steady statecoefficient of friction is initially 0.15, then decreases to 0.09 atapproximately 200 s. This value is maintained from approximately 200 sto the conclusion of the test. The consistent lubricating ability of thecomposite coating is attributed to two factors: the native mechanicalstrength of graphene, with Young modulus of 1 TPa and intrinsic stressstrength of 130 GPa; and the strong adhesive effects of zinc oxide andPVDF. The high-bearing load of the composite coating validates itspotential application for high pressure loads.

To quantify endurance of the composite coating under high load, thecoefficient of friction was measured to sliding distances of 145, 300,and 450 m under normal load 10 N. FIG. 8 shows effect of slidingdistance on friction reduction. In FIG. 8, X-axis numbers indicate themaximum sliding distance in meters that particular measurement was ranand the tribocoating survived. During unlubricated steel-steel sliding,the coefficient of friction rapidly increases to 0.9 after approximately25 s, then remains constant with unsteady behavior to a sliding distanceof 150 m. During lubricated sliding, the coefficient of frictionincreases to a maximum of only 0.17, then remains at 0.1 to long slidingdistances 150 and 300 m. At a sliding distance 450 m, frictioncoefficient of the composite coating increases from 0.1 to 0.4 with poorstability. This is attributed to generation of wear particles at thesliding surfaces after natural degradation of the coating. However, evenafter a long sliding distance of 450 m under 10 N load (Hertz contactpressure 0.89 GPa), the frictional losses using the composite coating isstill 50% less than that of the dry unlubricated configuration. Whilefriction reduction is maintained to a maximum service distance of atleast 450 m, a critical distance of approximately 300 m defines asignificant departure in the lubrication ability at the interface of thesliding surfaces.

In addition to reducing friction coefficient, the composite film of thisdisclosure decreases both wear track formation and surface roughening.After testing with normal load 10 N, optical micrographs and 3D surfacescans of the ball specimen indicate a change in lubrication effectsafter a critical distance of 300 m as shown in FIG. 9. FIG. 9 shows 3Dsurface reconstruction of sliding distance-dependent wear tracks on themoving specimens, done by computational geometry algorithm used torecreating a surface from scattered data points. On the stationaryspecimens, lubricated sliding enables wear protection up toapproximately 300 m but continues to prevent excessive roughness.Referring to FIG. 9, unlubricated sliding to 145 m produces a deep,rough track as seen in micrograph labeled “a”; lubricated sliding up to145 m does not show formation of an appreciable wear track as seen inmicrograph labeled “b”; lubricated sliding to 300 m produces adeveloping, smooth wear track as seen in micrograph labeled “c”; andlubricated sliding to 450 produces a shallow, smooth wear track on thecontact surface as seen in micrograph labeled “d”. After unlubricatedsliding to 145 m, the formed wear track has diameter 656 μm androughness 540 nm. After lubricated sliding to 145 m, the contact surfacedoes not form an appreciable wear track; this result is consistent toprevious wear observations. After lubricated sliding to 300 m, thecontact surface begins to develop slight wear with diameter 135 androughness 50 nm. After lubricated sliding to 450 m, an appreciable trackis observed with diameter 494 μm and roughness 56 nm. Interestingly, thewear track developed after lubricated sliding to 450 m is 25% smallerbut 90% smoother than that developed after unlubricated sliding. Theperseverance of a smooth surface after 300 m to 450 m further evidencesa persistence of graphene around the wear track even after slidingdistance 300 m.

Sliding distance, to a critical value, has a significant influence onfriction reduction and wear track formation on the stationary ballspecimen. However, with graphene-based composite lubricant, slidingdistance does not influence formation of the wear features on therotating disk specimen. FIG. 10 shows 3D surface reconstructions (asmentioned above) of sliding distance-dependent wear track on tracks inon the rotating specimens. In FIG. 10, micrograph labeled “a” showsunlubricated sliding up to 150 m produces a deep, rough track. Incontrast, micrographs labeled “b”, “c’ and “d” corresponding tolubricated sliding up to 150 m, 300 m, and 450 m respectively do notshow formation of an appreciable wear track. Referring to FIG. 10, aftertesting with normal load 10 N, optical micrographs and 3D surfacereconstructions of the rotating disk specimen show no appreciable weartrack after 450 m in lubricated sliding. After unlubricated sliding to145 m sliding distance, a wear track with 0.4 μm in depth and about 650μm in width was formed on the disk specimen. After lubricated sliding to145 m sliding distance, the contact surface does not form an appreciablewear track. This observation is consistent for lubricated sliding to 300m and 450 m. In all surface reconstructions for lubricated sliding, theintense contour features adjacent to either side of the wear track aredue to solid particles produced from the graphene-based composite film.

The high graphene content of the wear track on the stationary ballspecimen suggests the mechanical and chemical durability of thecomposite lubricant. The wear track was formed after application ofnormal load 10 N to sliding distance 145 m. Identified using Ramanspectroscopy, the remaining composite film is tracked by the intensevibration mode of graphene at frequency 1580 cm⁻¹. FIGS. 11A through 11Dshows Raman spectral maps of stationary surface specimen. FIGS. 11A and11B show that the intense carbon signal following tribology testingcorrelates to the visible graphene coating that remains on the surfaceof the ball specimen. FIGS. 11C and 11 D show that after removing thevisible layer, appreciable carbon signal remains on the surface of theball specimen. This suggests the importance of a zinc-based bindingagent between the carbonaceous graphene and the stainless steel surface.The digital photograph of the wear track after tribology testing shows adark opaque film covering the specimen surface. Correlation of thespectral color map to the photograph indicates the protective filmcontains high graphene content. In addition, scratches both within andsurrounding the wear track contain high graphene content. It istheorized that this persistent film prevents excessive wear on the ballsurfaces during sliding, resulting in friction reduction. After removingthe film, the digital photograph of the wear track does not show thedark opaque coating; however, the spectral color map indicates that theedges of the wear track and the surrounding scratches retain high carboncontent. The persistence of graphene around the wear track is attributedto the zinc oxide additive: the zinc-containing compound serves as abinding agent between graphene and the stainless steel surface.

FIGS. 12A and 12B show Raman spectral maps of disk substrate surface attwo different locations. Both maps show that following tribologytesting, appreciable carbon signal correlates to the scratches thatadorn the wear track formed on the rotating specimen. The intense carbonsignal on either side of the wear track are due to residual surfaceparticles from the composite coating film. Similarly, the high graphenecontent of the wear track on the rotating disk specimen evidences thedurability of the composite film. The wear track on the disk was formedafter application of normal load 10 N to sliding distance 145 m. Thedigital photograph of the wear track after tribology testing shows arectangular track populated by linear scratches (machining marks) andblack particles (FIGS. 12 A and 12B). The edges of the track are adornedby more small black particles; these materials are attributed to thedecomposition of the composite film. The spectral map indicates therandomly-oriented scratches on the wear track retain appreciable carboncontent after tribology testing. Interestingly, the sliding trackremains wholly undamaged: aside from surface scratches, no significantindicators of wear are observed on the disk surface.

Raman spectral mapping of the ball and disk specimens after tribologytesting provided evidence that the graphene-rich composite film isretained under extreme operating conditions. Furthermore, thepersistence of the film after testing evidences the importance of azinc-based binding agent between the graphitic carbon and the contactsurface.

The graphene-rich composite of this disclosure has proven exceptionalperformance as a solid-state lubricant under high contact pressure.Tribology testing, under applied loads up to 15 N (Hertz contactpressure 1.02 GPa), demonstrates the composite film retains frictionreduction for at least 150 m with stable behavior. Further testing,under constant load and different sliding distances, evidences thesignificance of zinc oxide to frictionaol.com reduction and wear trackpreventing or smoothing. Characterization before tribotesting suggeststhat the produced composite film is a physical mixture of itsprecursors, with no chemical or crystallographic modification resultingfrom the preparation process. Spectral mapping after tribotestingconfirms the persistence of the composite film on both the rotating andstationary contact surfaces. The durability and resilience of thegraphene-based coating prove its great potential as a solid lubricantfor dry sliding and high load-bearing applications.

In following sections, the materials and methods used to prepare thecomposite material solid lubricant of this disclosure, which has beentested producing the results presented above are described.

Synthesis of Zinc Oxide:Zinc oxide powder was prepared by calcination ofzinc acetate dihydrate (Sigma Aldrich Corp.). The zinc precursor wasloaded into a rectangular aluminum oxide crucible (MTI Corp.) and placedwithin a horizontal quartz tube furnace (MTI) under continuouscompressed air flow at a rate of approximately 100 mL min⁻¹. The furnacewas heated at uniform temperature rate 10° C. min⁻¹ to a dwelltemperature of 500° C. for 2 hours. After cooling to room temperature,the product was ground and homogenized using a mortar and pestle. Thecollected zinc oxide powder was utilized in the following procedureswithout further processing.

Preparation of Composite Coating: The solid-phase composite lubricantslurry was prepared by ultrasonic homogenization. A viscous mixture of85.5%-wt. graphene (United Nanotech Innovations PVT Ltd.), 9.5%-wt. zincoxide, and 5%-wt. polyvinylidene difluoride (PVDF, Sigma-Aldrich) wasprepared with solvent N-Methyl-2-pyrrolidone (NMP, Sigma-Aldrich).Homogenization was performed in a sealed borosilicate scintillation vial(Thermo-Fisher Scientific Co.) dispersed using an ultrasonic bath (RPICorp).

Lamination of the ultrasonically-mixed dispersion onto the contactsurface was performed using a spin-coating technique. The homogenizedmixture was transferred to the center of the stainless steel disksubstrate (Bruker Scientific Co.). Immediately, the loaded disk wasaccelerated to a constant rotational speed 1000 rev min⁻¹ using atribometer (UMT-3, Bruker Corp.). The disk was rotated at this speed forapproximately 2 min. After deceleration to rest, the disk surface wasuniformly covered by a black thin film (approximately 10 μm thick). Thecoated disk was then dried at temperature 80° C. for at least 12 hoursto remove the NMP solvent.

FIG. 13 is a schematic representation of synthesis of zinc oxide,graphene, and binder composite film of this disclosure. Zinc acetate,heated in a continuous air stream, is oxidized to zinc(II) oxide. Thisproduct is homogenized with graphene and polyvinylidene fluoride binderusing N-Methyl-2-pyrrolidone solvent. The resulting mixture is laminatedand dried to produce the solid lubricant film. (Note: the triangle inFIG. 13 denotes heat is being added)

To understand the tribological role of zinc oxide, two referencecomposite coatings were prepared: (I) graphene and PVDF, and (II)graphene only. The coatings were prepared using modified versions of theabove procedure. To produce the graphene and PVDF coating, a viscousmixture of 95%-wt. graphene and 5%-wt. PVDF was prepared with solventNMP (Sigma-Aldrich) using the ultrasonic dispersion technique previouslydescribed. To produce the graphene coating, 100%-wt. graphene wasultrasonically homogenized with solvent NMP. Lamination of the referencecomposites onto the disk substrate were performed using the spin-coatingtechnique previously described.

X-Ray powder diffraction (XRD) was performed with an X-raydiffractometer (Smartlab III, Rigaku Corp.) with a cross-beam opticssystem. For powder materials, approximately 2 mg of graphene, zincoxide, or PVDF were packed into the cavity of borosilicate sampleholders (Rigaku) to packing depth approximately 2 mm. For the compositecoating, the mixed composite dispersion was dried inside the cavity attemperature 80° C. in vacuo. Loaded sample holders were then mountedinto the theta-theta goniometer (Rigaku). Monochromatic Cu-Kα radiationwas produced with a 9 kW rotating anode X-ray source, and collected witha sodium iodide scintillation detector (Rigaku). Spectral patterns wereproduced in the 2θ scattering angle range 2-150° at scanning rate 0.5°min⁻¹. Reported spectral patterns are smoothed for clarity ofinterpretation, but not reduced for background. For diffractograms ofmaterials containing graphene, the intense (002) feature has beentruncated to improve visibility of less intense features.

Scanning Electron Microscopy (SEM) was performed using a dual-beamscanning electron microscope (Quanta 3D FEG, FEI Co.). For powdermaterials, approximately 2 mg of graphene, zinc oxide, or PVDF wereadhered to an aluminum sample stage using double-sided carbon tape (3MCorp). For the composite coating, the mixed dispersion was dried on thestage at temperature 80° C. in vacuo. Loaded sample stages were placedinside the microscope chamber and evacuated to high vacuum (i.e., <2.6nbar). Micrographs were recorded at various magnifications afterthorough optimization of electron beam alignment, stigmation, focus,brightness, and contrast. Energy dispersive X-ray spectroscopy (EDXS)was performed using a 80 mm² area silicon drift detector (OxfordInstruments PLC) at energy level 10 keV. Electron pixel maps wereproduced using the AZTEC analysis software suite (Oxford Instruments).

Thermogravimetric Analysis (TGA) was performed using a simultaneousthermal analyzer (Q600, TA Instruments Inc). Approximately 4 mg zincacetate dehydrate were loaded into a cylindrical aluminum oxide crucible(TA Instruments). The weight of the crucible was tared prior to sampleloading. The loaded crucible was placed inside the horizontal furnacechamber under continuous compressed air flow at rate 100 mL min⁻¹.Sample mass was recorded during heating at uniform temperature rate 10°C. min⁻¹ to temperature 1000° C. Reported differential thermograms aresmoothed for clarity of interpretation.

Raman Spectroscopy was performed using a Raman microscope (DXR,Thermo-Fisher Scientific). The apparatus was calibrated using apolystyrene calibration standard (Thermo-Fisher). For powder materials,approximately 2 mg of graphene, zinc oxide, or PVDF powders were evenlydispersed across a borosilicate microscope slide (Fisher). For thecomposite coating, the mixed dispersion was dried on the slide attemperature 80° C. in vacuo. The loaded slide was then placed inside themicroscope chamber. Spectral patterns were produced using an aperaturedgreen laser with wavelength 532 nm, beam diameter 25 μm, and power 8 mW.A single spectral pattern is the average of at least 3 exposures, with acollection time of 20 seconds per exposure. Reported spectral patternsare smoothed and background-reduced for clarity of interpretation. Theareal D-G intensity ratio R_(D/G), a relative measure of sp² and sp^(a)hybridized carbon, is calculated as the ratio of the area of the D modespectral peak A_(D) to the area of the G mode spectral peak A_(G) Ramanspectra were de-convoluted into constituent spectral peaks by fittingeach excitation feature to the pseudo-Voigt function.

Following tribology testing, ex situ Raman spectral maps were collectedfor the ball and disk specimens after sliding contact under appliednormal load 10 N to a sliding distance 145 m. The wear track on the ballspecimen was characterized both before and after removing the visiblesurface film from the wear track. The film was removed by gentlysweeping a dry fiber cloth across the specimen surface. Each Ramanspectral map was produced from at least 169 sampled points representedas a color-scaled cluster map. Each Raman point spectrum was producedwith aperatured beam diameter 1.5 μm, and vertical and horizontal stepsize 2.5 μm. All color maps represent the spectral intensity atexcitation frequency 1580 cm⁻¹.

Tribological testing was performed at a steel-steel interface using auniversal mechanical tribometer (UMT-3, Bruker Corp.). An opticalsurface profilometer was used to measure the arithmetic average surfaceroughness R_(a) of the specimens and wear measurements of the testedspecimens. Tribological performance was measured at ambient conditions(i.e., 27° C. and 1 atm) in the ball-on-disk configuration: friction andwear were measured during pure sliding contact between the stationaryball and the rotating disk. The stationary specimen was a stainlesssteel ball with diameter of 6.3 mm and surface roughness R_(a) of 60 nm,and the rotating specimen was a stainless steel disk with diameter of 70mm and surface roughness R_(a) of 20 nm. Applied normal load was variedfrom 5-15 N (average Hertz contact pressure 0.71-1.02 GPa) and slidingdistance was varied from 150-450 m. Tribology tests were repeated atleast three times with error of measured friction and wear below 5%.Prior to testing, all specimens were cleaned with anhydrous acetone(Sigma Aldrich) to remove surface contamination.

In further experiments, another binder formulation in place of PDVF wasutilized. Aremco-Bond™ 570 polymer-graphene-zinc oxide composite wasutilized to increase the adhesion and durability of the coating oncontact surfaces and to reduce friction and wear in bearing steels underhigh contact pressure. The composite was made from graphene, zinc oxideparticles, and Aremco-Bond™ 570 polymer as binder. This binder containsbutanone, isopropanol, phenol, carbon black, formaldehyde, and o-cresol.The composite coating, with an approximate thickness of 15 μm, waslaminated on ASTM 52100 Bearing alloy Steel or also called as ChromeSteel” 52100 bearing steel discs. A sliding wear test with aball-on-disc configuration was used to measure the tribologicalperformance of the composite coating under a contact pressure of 1 GPa.It was demonstrated that friction and wear on the coated surface werereduced significantly compared to the uncoated surface. The surfaceadhesion properties of the coating were measured using the Nanovea®scratch tester and compared to an earlier graphene-zinc oxide coatingand found improved adhesion. FIG. 14 shows a comparison of thecoefficient of friction of a surface obtained by using this compositelubricant utilizing this binder Aremco-Bond 570) containing butanone,isopropanol, phenol, carbon black, formaldehyde, and o-cresol and anunlubricated surface. Referring to FIG. 14, the tribological performanceof the solid lubricant-coated surface was superior to that of theuncoated surface. Under a normal load of 4 N and duration of 3000cycles, the coefficient of friction declined about 82% in thegraphene-based coating compared to the uncoated contact. The coefficientof friction in the uncoated surface started at 0.69, and after 500cycles, rose to the steady state value of 0.84. In contrast, thecoefficient of friction in the coated surfaces started at 0.14 andreached the steady state of 0.15, exhibiting more stable behavior duringthe test.

To evaluate the influence of normal load on the tribological behavior ofthe composite coating, the coefficient of friction in three differenttests were measured with normal loads of 4, 8, and 12 N corresponding toHertzian pressures of 1.01, 1.27, and 1.45 GPa, respectively. FIG. 15shows effect of normal load conditions on the coefficient of friction ofa surface coated with a composite coating of this disclosure using acomposite coating utilizing Aremco-Bond 570 (containing butanone,isopropanol, phenol, carbon black, formaldehyde, and o-cresol) as thebinder in the composite. Referring to FIG. 15, the coefficient offriction remained almost constant for the different loads up to 500cycles. The 12 N normal force test resulted in faster utilization of thecoating such that after 500 cycles, the coefficient of friction startedto increase. However, the test with normal force of 8 N lasted longer(more than 1000 cycles). Therefore, it can be concluded that as theamount of load increases, the durability of the coating diminishes dueto the higher wear rate at the higher normal load. The consistency ofthe solid lubricant is related to the native mechanical strength ofgraphene and the strong adhesive properties of zinc oxide and thebinder. The solid lubricant's high-bearing load capacity demonstratesits potential for high-pressure applications.

Thus, in this disclosure, a novel graphene-zinc oxide composite film iscreated and studied as a solid-state lubricant for friction and wearreduction under extreme load conditions. The liquid-free composite ismade from a slurry of graphene, zinc oxide, and polyvinylidenedifluoride spin-coated onto a stainless steel substrate. Enhancedtribological performance was measured under ambient conditions using aball-on-disk tribometer with contact pressures up to 1.02 GPa andsliding distances up to 450 m. The graphene-rich lubricant demonstratessubstantial friction reduction and wear loss (approximately 90%)compared to unlubricated sliding. The composite film is able to maintainits lubricating effects under extreme operating conditions including 15N normal load and 450 m sliding distance. Following tribologicaltesting, optical and spectroscopic analysis of the formed wear tracksreveal a persistent protective film on the ball and disk surfaces. Theexcellent tribological performance of this graphene-rich composite isattributed to the adhesion effect from zinc oxide: zinc adheres grapheneto the contact interface, maintaining improved tribological performanceunder high contact pressure. The durability and resilience of thisadhesive coating suggest exceptional potential as a dry lubricant forhigh load-bearing applications.

Based on the above detailed description, it is an objective of thisdisclosure to describe a solid-state lubricant composition comprisinggraphene, an oxide of a metal, and a polymeric binder In such asolid-state lubricant, the weight percent of graphene can be in therange of 70 to 90, a non-limiting preferred range being 80 to 85. Forthe solid-state lubricants of this disclosure, the oxide can be an oxideof a metal, such as, but not limited to zinc, tin, molybdenum, silver,copper, lead, indium and antimony. In a preferred composition, the oxideof a metal was zinc oxide with weight percent in the range of 5 to 20, apreferred range being 10 to 15. Further, non-limiting examples suitablefor use as the polymeric binder in the solid-state lubricants of thisdisclosure are polyvinylidene difluoride, and polyethylene oxide,polyvinyl acetate, and polytetrafluoroethylene. In some embodiments ofthe solid-state lubricant of this disclosure, the polymeric bindercomprises butanone, isopropanol, phenol, carbon black, formaldehyde, ando-cresol. A non-limiting range for the weight percent of the polymericbinder in the solid-state lubricants of this disclosure is 2 to 10, witha preferred range being 5 to 8. It should be noted that in thepreparation of the lubricant composition an organic solvent is alsotypically used which later evaporates in processing as described above.

It should be noted that in some embodiments of the solid-lubricant ofthis disclosure the metal oxide can be replaced by sulfides, nitrides orfluorides of a metal. Examples of such replacements for the metal oxideinclude, but not limited to Molybdenum Sulfide (MoS₂). Tungsten sulfide(WS₂), boron nitride and titanium nitride. It should also be furthernoted that in some embodiments of the solid lubricants of thisdisclosure, more than one polymeric binder can be incorporated. Thus thesolid-lubricants of this disclosure can contain one or more than onepolymeric binder.

Based on the above description, it is another objective of thisdisclosure to describe a method of making a solid-state lubricantcoating on a substrate. The method includes the steps of making ahomogeneous slurry comprising powder of an oxide of a metal, graphene,and a polymeric binder; coating a substrate with the homogeneous slurry;and drying the slurry on the substrate, resulting in a solid lubricantcoating on the substrate. In some embodiments of the method, thehomogeneous slurry is made using sonication. In some embodiments of themethod, the coating the substrate is produced by a spin coating process.In some embodiments of the method the substrate is made of stainlesssteel. It should be recognized that other substrates can be used such aspolymers, woods, alloys. The substrate can be a metal, plastic, wood oran alloy. Examples of other metals and alloys that can be used assubstrates include, but not limited to, nichrome, Haynes 230 alloy,plastics such as Styrofoam™, low density polyethylene etc. In someembodiments of the method, the polymeric binder is one of polyvinylidenedifluoride, polyethylene oxide, polyvinyl acetate. In some embodimentsof the method, the polymeric binder contains butanone, isopropanol,phenol, carbon black, formaldehyde, and o-cresol. In some embodiments ofthe method, the weight percent of the polymeric binder is in the rangeof 5 to 8. In some embodiments of the method, the metal is one of zinc,tin, molybdenum, lithium, cobalt and antimony but is not limited tothese examples. In a preferred embodiments of the method, the metal iszinc.

Based on the above detailed description, it is yet another objective ofthis disclosure to describe another method of making a solid-statelubricant coating on a substrate. The method includes first making ahomogeneous slurry comprising zinc oxide powder, graphene,polyvinylidene difluoride, and an organic solvent. Then, a substrate iscoated with the homogeneous slurry. Solvents suitable for this purposeinclude but not limited to include acetone, ethanol, hexadecane,propanol, N, N-dimethylformamide, N-methyl-2-pyrrolidone, and ethyleneglycol. The slurry is then dried on the substrate, resulting in a solidlubricant coating on the substrate. A non-limiting method of making thehomogeneous slurry is sonication. Other methods include, but not limitedto mixing or stirring. The coating on the substrate can be accomplishedby spin coating. Other methods include, but not limited to spray coatingand dip coating. In the method, a non-limiting example of a substratethat can be used is stainless steel, which is but one example of contactsurfaces wherein it is desired to reduce friction, and for which thesolid-lubricants of this disclosure are applicable. Other substratematerials include, but not limited to, other metallic substrates andceramic substrates. It should be recognized that in this method, morethan one polymeric binder can be employed. Likewise, more than oneorganic solvent can be employed.

The dry solid lubricants of this disclosure find applications in manyfields that require reduction of frictional forces. Some areas where thesolid lubricants of this disclosure find industrial applicationsinclude, but not limited to: molding, including injection molding (forexample as release agents); cutting tools; chains; pistons; foodpackaging industry; railway track joints; machine shop works; locks;open gears; air compressors; gears and oxidizing environments. The solidlubricants of this disclosure are also useful at high temperatures,extreme contact pressures, places where fretting and galling is aproblem (e.g. bearings). Thus it is another objective of this disclosureto describe machines and apparatuses that utilize the solid lubricantsof this disclosure.

While the present disclosure has been described with reference tocertain embodiments, it will be apparent to those of ordinary skill inthe art that other embodiments and implementations are possible that arewithin the scope of the present disclosure without departing from thespirit and scope of the present disclosure. Thus, the implementationsshould not be limited to the particular limitations described. Otherimplementations may be possible. It is therefore intended that theforegoing detailed description be regarded as illustrative rather thanlimiting. Thus, this disclosure is limited only by the following claims.

1. A method of making a solid-state lubricant coating on a substrate,the method comprising: making a homogeneous slurry comprising powder ofan oxide of a metal, graphene, and a polymeric binder; coating asubstrate with the homogeneous slurry; and drying the slurry on thesubstrate, resulting in a solid lubricant coating on the substrate. 2.The method of claim 1, wherein the homogeneous slurry is made usingsonication.
 3. The method of claim 1, wherein the coating the substratewas done by spin coating process.
 4. The method of claim 1, wherein thesubstrate is made of stainless steel.
 5. The method of claim 1, whereinthe polymeric binder is one of polyvinylidene difluoride, polyethyleneoxide, polyvinyl acetate,
 6. The method of claim 1, wherein thepolymeric binder comprises butanone, isopropanol, phenol, carbon black,formaldehyde, and o-cresol.
 7. The method of claim 1, wherein the weightpercent of the polymeric binder is in the range of 5 to
 8. 8. The methodof claim 1, wherein the metal is one of zinc, tin, molybdenum, lithium,cobalt and antimony.
 9. The method of claim 8, wherein the metal iszinc.